1. Field of the Invention
This invention relates generally to equipment for measuring the impact resistance of different materials, and more particular, to a dropped-weight testing assembly for testing the failure energy of rigid materials.
2. Description of the Related Art
The use of plastic, aluminum or other sheeting materials for structural applications is increasing due to weathering, cost and weight advantages provided by these materials. The quality for a particular batch of sheeting material, however, varies and thus it is necessary to test whether a predetermined minimum impact resistance has been attained. In order to accurately determine the energy required to crack or break these rigid sheeting or flat sections used in building products, dropped-weight testing apparatus and procedures (hereinafter vertical height impact tests) have been developed. These apparatus and procedures call for the dropping of a weight, of given size and material, onto a specimen from various heights to determine the energy of a ductile-to-brittle transition for the specimen. This test has been standardized and is specifically set forth in ASTM D4226-99a, and for rigid PVC siding, ASTM 3679-99a, both of which are incorporated herein by reference.
A critical component of the vertical height impact test is the vertical alignment of the dropped-weight on the dart, impactor or tup (hereinafter xe2x80x9cdartxe2x80x9d), and the perpendicular direction of the force translated to the specimen by the dart. Off-center alignment will cause the specimen to be sheared and not stretched to failure. Further, non-perpendicular impact focuses the test energies into a smaller area of the test specimen at a higher value.
Present vertical heigh impact test apparatus have design configurations that exacerbate the potential inaccuracy in failure energy results for a given test material conducted under the same conditions. Premature failure of a substrate is very common due to the improper alignment of impact mechanisms. One of the sources for this premature failure is the ability of the dropped-weight assembly to travel non-linearly during free-fall. Typical vertical impact testing apparatus use a barrel to guide the dropped-weight assembly to the dart. Clearance is provided between the barrel and the dart in an attempt to reduce friction. This clearance, however, allows the dropped-weight assembly to impact the dart at a non-perpendicular angle. Further, since these apparatus are often manually operated, the clearance between the barrel and dropped-weight assembly also allows the operator to unknowingly shift the dropped-weight assembly to an off-center alignment with the dart. If the dropped-weight assembly impacts the dart in a non-centered position, the dart enters the test specimen at an angle thereby focusing the test on a smaller than expected area. This results in premature failure of the test material and an inaccurate impact resistance determination.
Another source of variation commonly encountered is the inability of the dart to remain in vertical alignment during impact by the dropped-weight assembly. This arises from the support mechanism for the dart being slightly flexible. As a result, if the weight-assembly contacts the dart at a slight off-center position, the support may flex and permit the dart to enter the test specimen at an angle. Present vertical impact test structures use a lightweight support for the dart which has the advantage of being easy to move or transport. But such structure lacks sufficient rigidity to maintain the dart in vertical alignment for the bore that guides the dart during impact.
Further variation in the test equipment performance arises from the dart failing to remain vertically aligned through travel to the test specimen. Any lateral movement or tilt of the dart with respect to the vertical axis of the bore for the dart results in inaccurate impact strength results. This displacement is a function of both the amount of clearance between the dart and the inner diameter of the anvil cavity for receiving the dart, the clearance between the dart and the support arm, and the distance from the upper edges of the anvil hole and the lower edges of the support arm.
The use of a barrel or tube as a guide also leads to inaccurate results. In this design, the dropped-weight assembly must force the air through the end of the barrel and draw air in behind the weight assembly. This results in an additional loss of energy.
Thus it is desired to have an impact resistance testing apparatus that is specifically designed to be on a defined centerline and to maintain this centerline alignment during the testing of a test specimen. This will ensure that the dart contacts the test specimen at a perpendicular angle and that impact resistance readings are more consistent and more accurate. By eliminating the lateral movement of the weight assembly and the dart upon impact, negative variation is greatly reduced and a more accurate measurement of impact resistance of a given material can be determined.
The principal objects and advantages of the present invention include: providing a testing apparatus for accurate measurement of impact resistance of a test specimen; providing such a device that reduces the amount of variation in test results when conducted on a single test specimen under the same conditions; providing such a device that is optimized for testing a variety of sheeting materials, such as PVC, other plastics, aluminum and other sheet building material; and providing such a device that is easy to use and efficient in operation.
An impact resistance testing apparatus of the present invention comprises a rigid frame, an anvil connected to the lower region of the frame upon which a specimen will be placed for testing, a dart slidably positioned above the anvil to contact the specimen, and a dropped-weight mechanism positioned above the dart to apply a vertical impact force on the dart. The rigid frame comprises a vertical column, a guide rail and base. The guide rail is connected to, and aligned with, the vertical column to facilitate proper movement of the dropped-weight mechanism. To ensure that the dart contacts the specimen at a perpendicular angle, a braced support arm is connected to the frame above the anvil and below the dropped-weight mechanism. This support arm is designed to minimize flexure to ensure that the dart remains in vertical alignment upon impact from the slide hammer. A bearing is contained within a vertically oriented bore in the support arm and surrounds the dart for a sufficient length as to minimize lateral movement during vertical displacement of the dart and contact with the substrate.
The dropped-weight mechanism comprises a guide block and a weight assembly. The guide block is rigidly mounted to the weight assembly and slidably connected to the guide rail to facilitate downward travel of the slide hammer at the proper vertical orientation and to prevent tilting of the slide hammer upon impact with the dart.
The present invention provides a testing apparatus with improved reliability in impact resistance test results by reducing or eliminating the sources of variation. By maintaining proper alignment of the weight assembly and the dart through impact and vertical displacement, the test specimen is impacted at a true perpendicular angle to eliminate premature shear failure and inherent variation in the testing apparatus. Thus, the impact resistance measurements taken are a more accurate determination of the true strength of the substrate sample.
Other advantages and components of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, which constitute a part of this specification and wherein are set forth exemplary embodiments of the present invention to illustrate various objects and features thereof.